1. Field of the Invention
The present invention relates to a bias circuit for a power amplifier and, more particularly, to a bias circuit including an emitter follower integrally formed on one GaAs chip for a power amplifier and capable of operating by a low external reference voltage.
2. Background Art
GaAs-heterojunction bipolar transistors (GaAs-HBTs) have recently come into wide use in power amplifiers for portable telephones including those in code division multiple access (CDMA) systems and power amplifiers for wireless LANs.
In Japanese Unexamined Patent Publication No. 2004-343244, a circuit formed as a conventional GaAs-HBT power amplifier module is disclosed.
FIGS. 7A and 7B show an example of a conventional GaAs-HBT power amplifier module. A portion of the module shown within the area surrounded by the broken line in FIG. 7A is formed on a GaAs chip. Portions other than the portion within the broken-line area are formed by chip parts and lines on the module circuit board.
The circuit shown in FIG. 7A has an RF signal input terminal (IN) and an RF signal output terminal (OUT). Tr1 and Tr2 denote an initial-stage heterojunction bipolar transistor (HBT) and a final-stage HBT, respectively. Vc1 and Vc2 denote collector power supply terminals for the transistors Tr1 and Tr2, respectively. Vcb denotes a power supply terminal of a bias circuit of Tr1 and Tr2. Vref denotes a reference voltage input terminal of the bias circuit. Rb1, Rb2, Rb12 and Rb22 denote resistors. C1 to C4, C21 to C23, Cd1, Cd2 and Cdb denote capacitors. L1 and L2 denote inductors. L11 and L21 to L23 denote lines having particular electrical lengths and functioning as inductors.
FIG. 7B shows a concrete example of a circuit configuration of the final-stage transistor Tr2 and a circuit portion for biasing the transistor Tr2. (The initial-stage transistor Tr1 and a circuit portion for biasing the transistor Tr1 have substantially the same circuit configuration.) Trb1 to Trb5 denote HBTs. Rbb1 to Rbb6 denote resistors. Vrefb denotes a reference voltage input terminal (a terminal to which a reference voltage is externally applied) of the bias circuit. Vcb denotes a collector power supply terminal for the bias circuit. Vbo denotes an output terminal of the bias circuit. This bias circuit operates so as to constantly maintain the idling current through Tr1 and Tr2 for the power amplifier (bias current when no RF signal input is supplied) under varying temperature.
For the normal operation of the bias circuit for the power amplifier shown in FIGS. 7A and 7B, a reference voltage Vref higher than twice the barrier voltage of the HBTs, typified by that in the two-stage cascaded section formed of Trb4 and Trb5 in a diode connection Tr or that in the two-stage cascaded section formed of the final-stage Tr2 and the Trb1, is required. That is, in a case where GaAs HBTs are used, a Vref of about 2.7 to 2.9 V is required from consideration of a barrier voltage of about 1.25 to 1.30 V and a voltage drop of about 0.2 to 0.3 V across the resistor Rbb1.
FIG. 8A shows an example of input/output characteristics of the power amplifier shown in FIGS. 7A and 7B. FIG. 8B shows an example of temperature dependence of idling current Icq2 through the final-stage Tr2 (operating current when no RF signal input is supplied). In a power amplifier for a portable telephone in which an idling current flows at all times like in CDMA use, the magnitude of the idling current largely influences the maximum talk time of the portable telephone. Therefore, circuit constants are designed so that the desired output power (about 26 to 28 dBm) is obtained at a low idling current value. The Trb1 forming the emitter follower is a device having the function of absorbing the difference between the low base current of the Tr2 when the idling current is low and the high base current of the Tr2 during high-output operation.
A concrete example of variation in Icq2 with respect to temperature in use of the bias circuit shown in FIG. 7B will be described. If Icq2 is about 25 mA at ordinary temperature, it is about 20 mA at a temperature in the range from a low temperature to 25° C., and is about 28 mA at a high temperature of 85° C. Thus, the circuit shown in FIG. 7B is capable of effectively limiting variation in Icq2 with respect to temperature and it is considered that there is no problem with the circuit in practice if Vref of about 2.8 to 2.9 is applied.
In the bias circuit shown in FIG. 7B, however, no idling current flows even at ordinary temperature when Vref is reduced to a value lower than about twice the barrier voltage of the HBTs, i.e., about 2.5 V. For this reason, the bias circuit shown in FIG. 7B cannot be used in practice without being modified. This tendency of the idling current to stop becomes stronger as the temperature decreases because the barrier voltage determined by device materials is increased as the temperature decreases. The temperature dependence of the barrier voltage ordinarily has a gradient of about −1 to −2 mV/° C.
With the conventional bias circuit for a GaAs-HBT power amplifier, there has been a problem that the idling current does not flow at ordinary or low temperature when the reference voltage Vref is reduced to a value lower than about twice the barrier voltage of the HBTs, as described above.